The present invention relates to the field of mass storage devices. More particularly, this invention relates to a method and apparatus for screening disc drives to improve disc drive yield rates.
One key component of any computer system is a device to store data. Computer systems have many different places where data can be stored. One common place for storing massive amounts of data in a computer system is on a disc drive. The most basic parts of a disc drive are an information storage disc that is rotated, an actuator that moves a transducer to various locations over the disc, and electrical circuitry that is used to write and read data to and from the disc. The disc drive also includes circuitry for encoding data so that it can be successfully retrieved and written to the disc surface. A microprocessor controls most of the operations of the disc drive as well as passing the data back to the requesting computer and taking data from a requesting computer for storing to the disc.
The transducer is typically placed on a small ceramic block, also referred to as a slider, that is aerodynamically designed so that it flies over the disc. The slider is passed over the disc in a transducing relationship with the disc. Most sliders have an air-bearing surface (ABS) which includes rails and a cavity between the rails. When the disc rotates (generally, at rotational speeds of 5,400 RPM or higher), air is dragged between the rails and the disc surface causing pressure, which forces the head away from the disc. At the same time, the air rushing past the cavity or depression in the air-bearing surface produces a negative pressure area. The negative pressure or suction counteracts the pressure produced at the rails. The slider is also attached to a load spring, which produces a force on the slider directed toward the disc surface. The various forces on the slider equilibrate, so that the slider flies over the surface of the disc at a particular desired fly height. The fly height is the distance between the disc surface and the transducing head, which is typically the thickness of the air lubrication film. This film eliminates the friction and resulting wear that would occur if the transducing head and disc were in mechanical contact during disc rotation. In some disc drives, the slider passes through a layer of lubricant rather than flying over the surface of the disc.
Data is stored on the surface of the storage disc. Disc drive systems read and write information stored on tracks on storage discs. Transducers, in the form of read/write heads attached to the sliders, located on both sides of the storage disc, read and write information on the storage discs when the transducers are accurately positioned over one of the designated tracks on the surface of the storage disc. The transducer is also required to be moved to a target track. As the storage disc spins and the read/write head is accurately positioned above a target track, the read/write head can store data onto a track by writing data onto the storage disc. Similarly, reading data on a storage disc is accomplished by positioning the read/write head above a target track and reading the stored material on the storage disc. To write on or read from different tracks, the read/write head is moved radially across the tracks to a selected target track. The data is divided or grouped together on the tracks. In some disc drives, the tracks are a multiplicity of concentric circular tracks. In other disc drives, one track on one side of the disc drive is a continuous spiral. Each track on a disc surface in a disc drive is further divided into a number of short arcs called sectors. Servo feedback information is used to accurately locate the transducer head onto the tracks/sectors. The actuator assembly is moved to the required position and held very accurately during a read or write operation using the servo information.
The actuator assembly is composed of many parts that contribute to the performance required to accurately hold the read/write head in the proper position. There are two general types of actuator assemblies, a linear actuator and a rotary actuator. The rotary actuator includes a pivot assembly, an arm, a voice coil yoke assembly, and a head gimbal suspension assembly. The rotary actuator assembly pivots or rotates to reposition the transducer head over particular tracks on a disk. A suspension or load beam is part of the head gimbal suspension assembly. The rotary actuator assembly also includes a main body, which includes a shaft and bearing about which the rotary actuator assembly pivots. Attached to the main body are one or more arms. One or typically two head gimbal suspension assemblies are attached to the arm.
The actuator assembly is rotatably attached to a shaft via a bearing cartridge, which generally includes one or more sets of ball bearings. The shaft/post is attached to the base and may be attached to the top cover of the disc drive. A yoke is attached to the actuator assembly. The voice coil is attached to the yoke at one end of the rotary actuator assembly. The voice coil is part of a voice coil motor, which is used to rotate the actuator assembly including the attached transducer or transducers. A permanent magnet is attached to the base and cover of the disc drive. The voice coil motor, which drives the rotary actuator assembly, comprises the voice coil and the permanent magnet. The voice coil is attached to the rotary actuator assembly and the permanent magnet is fixed on the base. A yoke is generally used to attach the permanent magnet to the base and to direct the flux of the permanent magnet. Since the voice coil is sandwiched between the magnet and yoke assembly and is subjected to magnetic fields, electricity can be applied to the voice coil to drive it so as to position the transducers at a target track.
Tribological (mechanical resistance) characterization of the head-disc interface (head-actuator-arm-shaft-bearing cartridge) is generally critical to final qualification of both the shaft-bearing cartridges during the assembly operations of the disc drive. This is because the bearing cartridge is the only dynamic structure positioned between the actuator arm assembly and the base and further the bearing cartridge is the only dynamic structure holding the actuator arm assembly and the base in a disc drive. Generally, a lower preload applied between the bearing cartridge and the shaft causes lower mechanical resistance (axial mode resonance occurs at low axial mode frequency) and a higher preload applied between the bearing cartridge and the shaft causes higher mechanical resistance (axial mode resonance occurs at high axial mode frequency). Lower preload can cause seeking and settling problems and can affect the drive error rate performance, because lower preload generally introduces noise humps (in the range of 60-100 MHz) in the actuator arm during seeking, which can cause seeking and settling problems.
Current methods to characterize the preload applied to the bearing cartridge use a laser beam to shine on the cartridge during the seek operation to determine the generated axial mode frequency. The implementation of a laser beam method in a manufacturing environment generally needs sophisticated and elaborate calibration procedures. Also, the laser beam method requires expensive set-up and is generally very time consuming and not suitable for application in the manufacturing environment.
What is needed is a simpler, easier to set up, and less expensive method and apparatus to characterize the bearing cartridge based on the preload applied to the bearing cartridge in a manufacturing environment, to increase disc drive yield rates.
A method for characterizing a bearing cartridge based on a preload applied to the bearing cartridge used in a disc drive, to increase disc drive yield rates. The method begins with the step of loading the bearing cartridge on to a test fixture including a base and a voice coil motor-shaft-actuator assembly such that the voice coil motor-shaft-bearing cartridge-actuator assembly is rotatably attached to the base. Then, the method includes applying a voltage signal onto the voice coil motor to provide a steady swing to the actuator assembly including the shaft and the bearing cartridge. Then, the method includes measuring a voltage drop that occurs across the bearing cartridge during the steady swing of the actuator assembly to characterize the preload applied to the bearing cartridge. Then, the method further includes comparing the measured voltage drop with a predetermined threshold value to characterize the bearing cartridge.
Also discussed is a test fixture that includes a base, a voice coil motor-shaft-actuator assembly, and a bearing cartridge disposed between the shaft and the actuator assembly. The voice coil motor-shaft-bearing cartridge-actuator assembly is rotatably attached to the base. The test fixture can also include voice coil motor circuitry to apply a voltage signal to the voice coil motor to induce a steady swing of the actuator assembly. Further, the text fixture can include voltage divider circuitry to measure a voltage drop across the bearing cartridge during the steady swing to characterize the bearing cartridge based on the preload applied to the bearing cartridge to reduce seeking and settling problems during a seek operation and to increase disc drive yield rates during manufacturing.
Advantageously, the method and apparatus described above provides a simpler, a more easier to set up, and a less expensive method and apparatus to characterize the bearing cartridge based on the preload applied to the bearing cartridge in a disc drive during manufacturing, to increase disc drive yield rates.